1. Field of the Invention
The present invention relates to current-to-pressure (I/P) converters.
2. Description of the Prior Art
Various electro-pneumatic transducers which convert an electrical signal to a pressure signal for controlling valves and the like have been used in the prior art. Typically, these devices have used some form of mechanical feedback to stabilize the transducer at an output pressure commanded by the electrical signal.
Recently, a new type of I/P transducer or converter has been developed which operates based upon current flowing through a two-wire current loop and which uses electrical feedback signals representative of the output pressure along with the magnitude of the input current to control a transducer which varies the output pressure. This type of I/P converter is described in U.S. Pat. No. 4,481,967 by Roger L. Frick, which is assigned to the same assignee as the present application. Other examples of this new type of I/P converter are shown in the Prescott et al U.S. Pat. No. 4,492,246, the Simpson U.S. Pat. No. 4,527,583, and in U.K. patent application 2065331.
Typically, both the mechanical type electro-pneumatic converter as well as the newer electronic I/P converter are intended to be used in a process control environment where, in many cases, access to the I/P converter or the valve which it controls is difficult and time-consuming. When a malfunction or loss of operation occurs, there are a number of possible sources of the problem, only some of which involve the I/P converter. Among the causes of failure can be loss of input pressure, a restriction in the input line to the I/P converter, a stuck or contaminated valve, a plugged filter in the pneumatic line, improper pressure regulator settings, improper calibration of the I/P converter, or malfunction of the I/P converter. The ability to diagnose the cause of a system failure and to determine whether or not the I/P converter is the cause of that malfunction would be an extremely important improvement to the current state-of-the-art in I/P converters.
I/P converters are, in general, relatively low cost devices in comparison to, for example, a two-wire pressure transmitter which may be used in the same process control system. Improvements to I/P converters, therefore, most not involve significant increases in the cost of the I/P converter.